Ya I'd be taking a good look around heater-removable burner vent cover? Ive heard heater can foul the water but not to the point make your eyes water.
We had ours invaded with flock of little brown beetles years ago. I guess the radiant heat of the water-kept it from lighting-when got it to light wasnt a pleasant fragrance, though didnt last long. Most of em came out in a big whoosh
Windy-wonder if stuff got blown in-
Im assuming also it wasnt simply the way wind was moving fumes-ours will come in the kitchen window on occasion.